There are numerous inventions related to drill string telemetry, both by wire line and by wireless apparatuses. These include several inventions in which magnetic fields (near-field, induction) and low to high frequency electromagnetic waves (far-field, traveling waves) are a means of energy information transfer in or adjacent to a drill string.
U.S. Pat. No. 2,379,800, Signal Transmission System, by D. G. C. Hare, refer to the use of a transformer coupling at each pipe junction. One difficulty with the use of transformers is their high power requirements. U.S. Pat. No. 3,090,031, Signal Transmission System, by A. H. Lord, addresses these high power losses, and teaches the placement of an amplifier and a battery in each joint of pipe. However, the life of the battery became a critical consideration. In U.S. Pat. No. 4,215,426, Telemetry and Power Transmission For Enclosed Fluid Systems, by F. Klatt, an acoustic energy conversion unit is employed to convert acoustic energy into electrical power for powering the transformer junction.
Transformers operate upon Faraday's law of induction. Briefly, Faraday's law states that a time varying magnetic field produces an electromotive force which may establish a current in a suitable closed circuit. Faraday's law is:
emf=−dΦ)/dt Volts;
where emf is the electromotive force in volts, and dΦ/dt is the time rate of change of the magnetic flux. The negative sign is an indication that the emf is in such a direction as to produce a current whose flux, if added to the original flux, would reduce the magnitude of the emf. This principal is known as Lenz's Law.
An iron core transformer has two sets of windings wrapped about an iron core. The windings are electrically isolated, but magnetically coupled. Current flowing through one set of windings produces a magnetic flux that flows through the iron core and induces an emf in the second windings resulting in the flow of current in the second windings. The iron core itself can be analyzed as a magnetic circuit, in a manner similar to DC electrical circuit analysis. Some important differences exist, however, including the often nonlinear nature of ferromagnetic materials.
Briefly, magnetic materials have a reluctance to the flow of magnetic flux which is analogous to the resistance materials have to the flow of electric currents. Reluctance is a function of the length of a material, L, its cross section, S, and its permeability U.
Reluctance=L/(U*S) (ignoring the nonlinear nature of ferromagnetic materials).
Any air gaps that exist in the transformer's iron core present a great impediment to the flow of magnetic flux because iron has a permeability that exceeds that of air by a factor of roughly four thousand. Consequently, a great deal of energy is expended in relatively small air gaps in a transformer's iron core. See generally, HAYT: Engineering Electro-Magnetics, McGraw Hill, 1974 Third Edition, p. 305-312.
The transformer couplings revealed in the abovementioned patents operate as iron core transformers with two air gaps. The air gaps exist because the pipe sections must be severable. U.S. Pat. No. 4,605,268, Transformer Cable Connector, by R. Meador, utilizes closely aligned small toroidal coils to transmit data across a pipe junction.
The Shell Oil Company telemetry system includes a modified tubular member, having electrical contact rings in the mating surfaces of each tool joint. The contact rings in each tubular member are electrically coupled by an insulated electrical conductor extending between each contact ring. The insulated electrical conductor is disposed in a fluid-tight metal conduit to isolate the conductor from the fluid in and around the drill string when the tubular members are connected in a drill string and lowered in a well bore. The Shell Oil Company approach is described in U.S. Pat. No. 4,095,865, Telemetering Drill String with Piped Electrical Conductor. A different helical conduit is disclosed in Well Bore Data System, U.S. Pat. No. 4,788,544, the latter conduit being designed to adhere to the bore of each tubular member. Both approaches have several shortcomings.
Since it is difficult to secure the helical conduit to the bore wall of each tubular member, the helical conduit is secured to each tubular member only at the pin and box ends of each tubular member. As the tubular members are manipulated in the well bore, this helical conduit may respond by oscillating like a spring, causing the conduit to rub against the bore wall of the tubular members, which in time may produce a breach in the helical conduit. Drilling fluid will enter such a breach and impair the operation of the data transmission system.